You can't simply say
"well the germans have an immensely effective AT solution, so the russians need one too"
because that doesn't reflect the difficulties that they faced. The PTRD is in the game because it is effective against smaller tanks and armoured vehicles from the right position and shooting the right spot. Just because it can't kill things at amazing ranges and on the more well armoured spots on German tanks doesn't make it useless. It was also deployed against Anti-Tank guns, something I'm sure we're all hoping to see in RO at some time.
I'd like to see the russian anti tank grenades at some point